This invention relates to a method of forming an image by electrophotographic processes. More particularly, the invention relates to an image forming method that is advantageous for implementing electrophotographic processes using contact charging and/or impression development and/or impression transfer and/or impression cleaning.
The prior art apparatuses for forming an image by electrophotographic processes have heretofore adopted corona charging and corona transfer, but the use of such corona treatments has caused adverse effects on the environment by the ozone they generate. Hence, in order to reduce the amount of ozone generation, the use of contact charging and contact transfer instead of the conventional corona charging and corona transfer is currently under review. The benefits of contact charging and corona transfer are not limited to the advantage that small amounts of noxious ozone are generated; they can also be performed at lower voltages than corona charging and corona transfer and, hence, can advantageously be operated on a smaller amount of power.
For instance, according to the teaching of Unexamined Published Japanese Patent Application (kokai) No. Hei 3-293364, a release coat is provided on a contact charging member so as to prevent the filming of a toner on the latter; at the same time, the polyester base constituent of the toner is substituted by a N-containing radical, and a fluidity improving agent of low chargeability is externally added to the toner so as to prevent the saturation of the toner with charges, to improve its cleanability, and to prevent its filming on the contact charging member.
However, the volume average particle size of the toner used in this prior art technique is very small (.ltoreq.10 .mu.m). Therefore, if the amount of the external additive is increased up to about 2 wt % with a view to improving the flowability of the toner, only the external additive will pass uncaptured by an impression cleaning member such as a cleaning blade, and the adhering additive on the contact charging member will cause uneven charging, if not filming. This would make it impossible to create a desired pattern of a latent electrostatic image.
According to another prior art technique, such as the one described in Unexamined Published Japanese Patent Application (kokai) No. Hei 1-195459, a fluidity imparting agent such as hydrophobic silica is added externally to a toner, which is then used to improve the cleanability of a contact transfer roller.
However, not all toner particles that contact the impression transfer member in a non-papering mode will have the same polarity, and the particles that are picked up by the transfer roller under the applied cleaning bias will smudge the back side of the paper to be supplied in the next copy cycle. This phenomenon takes place not only due to the charging polarity of the toner but also in the case where the electric resistance of the toner is low enough to be susceptible to charge injection.
Unexamined Published Japanese Patent Application (kokai) No. Hei 3-121462 proposes that fine particles treated with silicone oil or silicone varnish be externally added in an amount of 0.05 to 3 parts by weight to 100 parts by weight of a toner, whereby the occurrence of "white void" is suppressed during contact transfer.
However, if the fine particles are added in amounts up to about 2 parts by weight, only the external additive will pass uncaptured by an impression cleaning member, such as a cleaning blade, and the adhering additive on the contact charging member will cause uneven charging, if not filming, and thereby make it impossible to create a desired pattern of a latent electrostatic image. As a further problem, an attempt to form an image by impression development will cause a developer fog if it is made under hot and humid conditions or if many prints are copied.
As the demand for higher resolution in an image has increased today, efforts are being made to reduce the size of toner particles, and, at the same time, apparatuses have been proposed that adopt a one-component nonmagnetic impression development method for enhancing the effect of a development electrode by minimizing the space for development.
For example, Unexamined Published Japanese Patent Application (kokai) No. Sho 63-279261 teaches a toner particle size distribution profile that permits rapid charging of the toner even if it is composed of small particles, and Unexamined Published Japanese Patent Application (kokai) No. Hei 2-262160 teaches a method of forming a fineline image by impression development using a toner whose 50% volume average particle size is no more than 8 .mu.m.
However, these prior art techniques have the disadvantage that fine toner particles no larger than 5 .mu.m, agglomerated toner particles or the external additive that has come off the toner particles, will adhere to the development member or the latent image carrier and appear as fog or white spots in the image. This phenomenon takes place not only due to the charging polarity of the toner but also in the case where the electric resistance of the toner is low enough to be susceptible to charge injection.